The ball screw has a screw shaft, nut, balls and a circulating mechanism. The screw shaft has a helical screw groove on its outer circumference. The nut has a helical screw groove on its inner circumference. The nut is mated with the screw shaft. The large number of balls are accommodated within a rolling track formed by the opposite screw grooves. The circulating mechanism enables the rolling track to act as a circulating track. It is used as a motion conversion mechanism to linearly drive the screw shaft by rotating the nut.
In general, there are various types of ball circulating mechanisms in the ball screw. One of which is a bridge-type mechanism. The bridge-type ball screw has a linking track for the screw grooves. The bridge members are mounted on a nut to make a rolling track a circulating track. Thus, its structure is relatively simple and compact.
In such a bridge-type ball screw, the bridge member, for circulating balls, is fit into a through hole formed in a cylindrical barrel of the nut. However, steps are caused in the ball transfer part that connects the nut screw grooves and the circulating grooves, formed as “S”-shaped curves, on an inner surface of the bridge member. These steps cause abnormal noise and thus reduce the life of the ball screw. A ball screw 51 is known and shown in FIG. 6 that can solve these problems. The ball screw 51 has a screw shaft 52 formed with a helical screw groove 52a on its outer circumference. A nut 53 has a screw groove 53a on its inner circumference and is mated with the screw shaft 52. A large number of balls 54 is accommodated within a rolling track formed by the opposite screw grooves 52a, 53a. Instead of the bridge members, this ball screw 51 uses a circulation groove (linking track) 53b formed by plastically working the inner circumference of the nut 53. Then, the screw groove 53a is formed by cutting with a rotary tool.
This prior art ball screw guides balls 54 between the nut screw groove 53a and the circulation grooves 53b without causing abnormal noise or torque variation (catching etc.). Thus, it suppresses life reduction (e.g., see JP2012-82961 A1).
However, this prior art method cannot avoid the generation of steps or edges due to the variation of the plastic working and cutting working. In addition, the formation of the screw groove 53a, by a rotary tool, needs a very long cycle time. Thus, it is not preferable due to the increase in the manufacturing cost.
Another ball screw 59 has been proposed as shown in FIG. 7. In this ball screw 59, balls 57 are set in a nut screw groove 56 and a bridge member 58. A circulation groove (linking groove) 58a is mounted on the nut 55. When assembling the ball screw 59, it is possible to efficiently eliminate the step between the nut screw groove 56 and the bridge circulation groove 58a by mounting the bridge member 58, coated with a coating layer 60, on a boundary surface 58b of the bridge member 58 forming a boundary with a surface 56a of the screw groove 56; engaging the nut 55 with a screw shaft; circulating the balls 57 between the screw groove 56 and the circulation groove 58a; and cutting protruded portion 60a, of the coating layer 60, into the screw groove 56 from the surface 56a of the screw groove 56 (e.g., see JP 2014-145463 A).
However, in the prior art ball screw 59, it is believed that the coating layer 60 would be peeled off by repeated passing of the rolling balls 57. Thus, the durability and reliability, as well as life of the ball screw, would be impaired. Additionally, abnormal noise would also be caused by the peeled debris.